Yes, Aldora. I was thinking in that direction ( where visibility for both values must be present 'original' and 'effective' ). So that I am aware of the full potential of this item I'm buying, and what is the performance I will be getting - at this point of time.

So the suggestion is great!

But I think showing the calculated 'difference' against the armor you're wearing gives you even greater idea whether to buy it or not.

When the item is highlighted, your character status window should be brought up as well and to show something like :-
Physical defence : 100 (5↑)
Magical defence : 50 (10↓)